Calorimeters have been used to measure the energy in a beam, usually by means of heating a working fluid and measuring the temperature rise within the fluid. One such prior art calorimeter is illustrated in an article by S. Lovell and Shun Shen, in Physics in Medicine and Biology, Vol. 21, No. 2, 1976 starting on page 198. This article illustrates a cylindrical calorimeter in which the beam to be measured enters along the axis of the cylinder and is absorbed by a working fluid therein. The fluid also flows from one end of the cylinder to another, flowing through a baffle maze. A second calorimeter is illustrated in Russian Pat. No. 270,299, in which the beam being measured enters along the axis of a circularly symmetric maze calorimeter. The symmetry of this calorimeter is that of surfaces rotated about an axis, but the calorimeter itself does not rotate.